patch 7.4.1638

Problem:    When binding a function to a dict the reference count is wrong.
Solution:   Decrement dict reference count, only reference the function when
            actually making a copy. (Ken Takata)

src/testdir/test_partial.vim

@@ -170,3 +170,13 @@ func Test_partial_string()
   let F = function('MyFunc', ['foo'], d)
   call assert_equal("function('MyFunc', ['foo'], {'one': 1})", string(F))
 endfunc
+
+func Test_func_unref()
+  let obj = {}
+  function! obj.func() abort
+  endfunction
+  let funcnumber = matchstr(string(obj.func), '^function(''\zs.\{-}\ze''')
+  call assert_true(exists('*{' . funcnumber . '}'))
+  unlet obj
+  call assert_false(exists('*{' . funcnumber . '}'))
+endfunc